1. Field of the Invention
This invention relates to an electrophotographic copying apparatus for effecting high-speed copying, and more particularly to an electrophotographic copying apparatus having an optical scanning device for continuously scanning an original image at high speed.
2. Description of the Prior Art
Heretofore, when a number of copies of an original were to be produced by an electrophotographic copying apparatus, the original was repetitively exposed to light to form an image on a photosensitive medium at each exposure, thereby providing a predetermined number of copies. For a number of copies to be produced at high speed, the exposure time must be shortened and the image formation speed increased. On the other hand, a novel electrophotographic method heretofore proposed and the advent of improved photosensitive medium have given rise to the possibility of retention copying which eliminates the necessity of exposing the original image to light each time in order to obtain a number of copies.
Retention copying is so called because an electrostatic latent image once formed by the exposure of an original image to light is retained for repeated use in the formation of copy images. During the repeated copy image formation, there is no need to carry out the latent image formation by the exposure of the original image to light and accordingly, the total copying time may of course be shortened. Again in this case, the time required for the original image to be exposed to light to form a latent image must be shortened for the speed-up of the copying operation. Also, the slit exposure system is suitable for high-speed copying, whatever type of electrophotographic process may be adopted. According to such system, the photosensitive medium in use is only required to secure an exposure area corresponding to the slit width and thus need not provide an area of plane for one whole image at the exposed location of the medium. This leads to the possibility that the photosensitive medium may assume a cylindrical or other desired configuration. In the slit exposure system, however, the optical system for scanning an original image scans the original during its forward stroke, and then must return to its initial position to start another scanning cycle. Such return to the initial position takes a finite period of time which, even if it could be reduced to the equal of or a fraction of the time required for the forward stroke, would be still limited in reduction by the requirement of mechanical accuracy and would form an obstacle against the speed-up of the copying operation.
The present invention solves the problems related to the above-noted points.